Hydrogen can be used as non-polluting fuel in fuel cells, internal combustion engines or turbines, as well as in any other system where gaseous hydrogen is used as a fuel. Obviously, power generating systems using hydrogen as a fuel need the hydrogen to be produced by a generating process. Generally, hydrogen produced by such generating processes is stored as a gas or as a liquid in tanks from where it is conveyed to the power generating systems (e.g. US2006/011659A1). These systems are equivalent to conventional transporting systems used for non-renewable fossil fuels.
There is a large number of hydrogen generating systems, such like inter alia catalytic reforming of hydrocarbons like ethanol as disclosed in U.S. Pat. No. 6,245,309-B1 and U.S. Pat. No. 6,461,408-B2, electrolysis of water as disclosed in US2004/0065542A1, hydrides as disclosed in US2005/0036941A1 and US2009/0274595A1, metals and acids as disclosed in U.S. Pat. No. 4,988,486, metals and alkalis or aminoboranes as disclosed in U.S. Pat. No. 7,052,658-B2.
According to the US Department of Energy (“DOE”), energy density is “the ratio of available energy per pound” i.e. per unit of weight (cf. Solar Glossary: http://www1.eere.energy.gov/solar/solar_glossary.html#E). The following table lists the energy of some substances that are of interest as fuels:
TABLE IEnergy densityEnergy densityby mass)by volumeSubstance(MJ/kg)(MJ/L)Liquid hydrogen143.0010.10Hydrogen gas compressed at 700 bar143.005.60Hydrogen gas143.000.01Lithium borohydride65.2043.40Methane (1.013 bar; 15° C.)*55.600.04Liquefied petroleum gas (“LPG”)19.6025.30propene*Liquefied petroleum gas (“LPG”)49.1027.70butane*Gasoline*46.4034.20Diesel*46.2037.30Lithium43.1023.00Kerosene*42.8033.00Magnesium24.7043.00Calcium15.9024.60Sodium13.3012.80Biodiesel*42.2033.00Lithium/sodium alloy (80/20)37.1420.96Lithium/magnesium alloy (80/20)39.4227.00Bioethanol*26.0035.60Hard coal*32.5072.40Soft coal*24.0020.00Wood*18.00Lithium-ion battery0.720.90Lead battery (automotive)0.140.36Substances marked * generate carbon dioxide when used as a fuel.
As apparent, the density by volume of hydrogen gas is extremely low so that storage thereof in vehicle tanks or stationary tanks raises efficiency problems. Therefore, ways to generate hydrogen in situ on demand have been searched for.
To be competitive with conventional fuels or electric batteries, the energy density of hydrogen-based propelling systems must be equivalent or higher. Chemical hydrogen generation offers this possibility. In addition to the herein above mentioned patent documents, further such systems are disclosed in U.S. Pat. No. 4,156,635, U.S. Pat. No. 4,498,927, U.S. Pat. No. 4,980,136 and US2006/0117659A1, as well as in U.S. Pat. No. 3,985,866.
U.S. Pat. No. 3,985,866 discloses a method of producing high-pressure hydrogen gas by reacting a fuel comprising aluminum as main component and alkali or alkali earth metal or alloys thereof as minority component, with water, in a pressurized argon atmosphere. The high-pressure gas is aimed for use as driving energy for turbines to propel small-sized self-propelling submarine bodies. Alkali metal and/or alkaline earth metal is added to lower the melting point of aluminum, and to enable an initial exothermic reaction with water that provides sufficient hydroxide to react with aluminum oxide and avoid passivation of metal aluminum which would prevent the reaction of the aluminum with water. The reactions underlying this method are violent, take place at very high temperatures and pressures, and are thus difficult to control. This renders the method disclosed in US-3985866 rather unfeasible in industrial practice.
A number of known chemical hydrogen-generating systems use processes metal or non-metal hydrides as well as reactions of metals with acids or alkalis. The following table compares a number of fuels, including gasoline and diesel as non-renewable fuels, when used in vehicle engines:
TABLE IIWeightTankof thevolumetankneededneededConsumptionforto storeEmissions(L/(kg/400 kmfuel for(gFuelEngine type100 km)L)(L)400 kmCO2/km)GasolineInternal85.843223.36170combustionDieselInternal65.12420.4110combustionLiquidInternal463.2618413.040hydrogencombustionLiquidFuel cell23.931.795.726.80hydrogenLithiumInternal21.3411.3185.3645.240combustionLithiumFuel cell11.115.8944.4423.560SodiumInternal38.6337.47154.52149.880combustionSodiumFuel cell20.1220.1280.4880.480
The majority of conventional hydrogen generating systems requires catalysts and/or ignition systems are expensive with regard to recycling the fuel or use highly toxic substances. Whilst a major proportion of these systems is susceptible of being installed in motor vehicles, they still are technically complex or involve technical, economical or environmental drawbacks, especially in respect of providing sufficient precision and sensitivity of generating a stream of hydrogen that may allow an immediate response to power demands as, for example, by direct injection thereof into an internal combustion motor or a turbine, and in respect of recycling fuel used. There was thus a need to develop a hydrogen generating system that would overcome these drawbacks.